This invention relates in general to an improved mandrel for transporting hollow cylinders and, more specifically, to a mandrel and process for using the mandrel for coating hollow cylinders.
Although this invention is especially useful for the fabrication of electrostatographic imaging members, it is not limited to such application. Electrostatographic imaging members are will known in the art and include electrophotographic imaging members and electrographic imaging members.
Electrophotographic imagining member may be in the shape of a hollow drum or cylinder and are coated with at least one active electrophotographic imaging layer. The active electrophotographic imaging layer may comprise a single photoconductive layer or comprise a plurality of active electrophotographic layers such as a charge generating layer and a charge transport layer. These drum shaped electrophotographic imaging members embodiments are well known in the art.
Electrostatographic drums are conventionally coated by immersing hollow cylinders into a liquid coating solution, withdrawing the cylinders coated with the coating solution and drying the coating on the cylinders. Generally, the coating applied to the cylinder is confined to the exterior surface of the cylinder to conserve coating material and to a) provide for electrical grounding of the device, b) provide for insertion and removal of end pieces, and c) allow for cooling and prevent heat build-up. The use of a mandrel that supports a hollow cylinder by gripping only the interior of the cylinder facilitates coating of the exterior surface of the cylinder without any mechanical object contacting either the exterior surface of the cylinder or the coating deposited on the exterior surface. The mandrel is supported at its upper end by any suitable conventional conveyor means. The conveyor means can comprise means to raise and lower the mandrel and/or the entire conveyor means may be raised and lowered by any suitable and conventional means such as an elevator means. To prevent any significant deposition of coating material onto the interior of the cylinder during immersion of the substrate into the coating bath, the cylinder axis is maintained in a vertical position or attitude and air within the hollow cylinder is trapped in at least the lower section of the interior of the cylinder by various known techniques. Trapping of the air in the cylinder while the cylinder axis is maintained in a vertical attitude minimizes wasteful deposition of coating material within the interior of the cylinder. One technique for trapping air within the cylinder is to insert the lower end of a mandrel into the upper open end of a cylinder, the mandrel having an expandable component positioned at or adjacent to its lower end which can be expanded to firmly contact and grip the interior of the cylinder to form a seal which traps air in the section of the cylinder below the seal during immersion of the cylinder in a coating liquid. One such technique is described in U.S. Pat. No. 4,680,246, the entire disclosure thereof being incorporated herein by reference.
Although expandable disk shaped members made of natural or synthetic rubber or elastomers such as natural rubber, EPDM, neoprene, butyl, nitrile or polyurethane perform well for a dip coating step, it has been found necessary that the freshly coated cylinder be supported on a metallic platen for transportation through a drying device or zone because the expandable disk or inflatable bladder tends to stick to the interior wall of a cylinder and inhibit or prevent mandrel removal after exposure to elevated temperatures during drying of the deposited coating. Also, the memory properties of the expandable disk or expandable bladder material degrade rapidly when repeatedly exposed to temporary compression or expansion forces and elevated drying temperatures. Degradation of the memory properties prevents the material from returning to its original shape and size after distortion. This, in turn, can cause difficulties in separating the disk or inflatable bladder from the interior wall of a cylinder.
Inflatable bladders also require a compressed fluid source, complex air tubing and couplings, air seals and the like which greatly increase the likelihood of failure during a coating or drying operation. For example, an air leak can cause a coated cylinder to fall away from a mandrel during or after coating. If a drum falls into a coating bath or onto the floor of a drying oven, the entire coating line must be shut down to remove the fallen drum and to repair the leak. Moreover, the large contact area between an inflated bladder and the interior of a cylinder presents heat transfer problems that ultimately result in coatings that have undesirable non-uniform physical and electrical characteristics following drying. When inflatable bladders are utilized to grip the interior of cylindrical substrates, difficulties are also encountered when the mandrel must travel over great distances because of the extended length of air feed lines required. When liquids are utilized to inflate an inflatable bladder, leakage of the inflating liquid into coating baths for cylindrical substrates requires shut down of an entire coating line and replacement of the entire coating.
In another embodiment of an expandable mandrel, the expandable mandrel component has a shape similar to that of a disk or thick washer. The outside diameter of this expandable disk shaped member, in its normal unstressed state, is slightly less than the interior diameter of the cylinder that is transported by the mandrel. The axis of this disk is coaxial with the main mandrel body. The expandable disk is expanded by applying compressive pressure on at least a segment of the upper and lower surfaces of the disk. The applied compression pressure causes the length of the circumference of the circular outermost edge of the disk to increase sufficiently whereby the outer periphery of the disk firmly contacts and grips the interior surface of the cylinder so that the mandrel can support and carry the cylinder from one location to another and also function as an air seal to trap air within the interior of the cylinder in the section below the cylinder when the cylinder is immersed in a liquid coating bath. The compressive pressure may be applied to the disk by applying a pulling or tension force on a tension shaft extending from a presser means through the center of the disk and through the mandrel body whereby the disk is squeezed by the presser means against the adjacent lower end of the mandrel body. Generally, the pulling or tension force is applied to the tension shaft by tightening a nut threaded onto the top end of the tension shaft. The nut applies a predetermined fixed compression pressure to the expandable disk shaped member. This arrangement performs well when the mandrel is utilized for transporting cylindrical substrates under of substantially constant temperature conditions such as during coating or cleaning operations. However, it has been found that where the same mandrel must transport a hollow cylinder from a zone at about ambient temperature to another zone at highly elevated temperatures such as a drying zone, it has been found that the original predetermined compression pressure changes due to softening of the elastomer in the expandable disk shaped member which reduces the gripping pressure between the disk shaped member and the interior surface of the hollow cylinder. This reduction in gripping force can diminish to the point where the drum falls away from the mandrel due to gravity and strikes the floor of the drying zone which in turn damages the sensitive photoconductive or other delicate coating on the drum. Even a small nick or scratch in the sensitive photoconductive surface of the photoreceptor renders it objectionable for use in electrophotographic copiers, printers and duplicators. If the initial gripping pressure of the expandable disk shaped member against the interior of the hollow cylinder is increased to compensate for the temperature induced change, the increased pressure can exceed acceptable limits which, in turn, can cause physical distortion of thin cylindrical substrates.